Compositional gradation of various materials has been extensively studied and practiced in a wide variety of fields. Further, many semiconductor based materials have attempted to use compositional grading in order to provide various benefits such as reduced lattice mismatch stress and other electronic effects related to grading of dopants. For example, LEDs are rapidly replacing lighting lamps on streets (e.g. traffic lights and automobile lights) and in homes, and also displacing CCFLs in illuminating LCD panels for computers and TV screens. Most LEDs are made of GaN crystals that are deposited on a sapphire substrate. There is a large lattice mismatch between these materials that causes high dislocation densities in GaN; however, industry currently has no better substrates. An ideal substrate for GaN would be SiC having a small lattice mismatch. Unfortunately, single crystal SiC (e.g. available from Cree Research) is currently only made by vapor crystallization which is both slow and expensive, e.g. the Lely method, and is typically limited to small wafer sizes, i.e. about 2 inches. In this approach, epitaxial growth of SiC single crystal on a single crystal silicon wafer includes using a common microwave enhanced plasma CVD reactor for depositing diamond and condensing SiC vapor at very high temperatures.
The most commonly available single crystal wafer used today is silicon. However, conventional CVD deposition of SiC on Si tends to form polycrystalline SiC due to spontaneous nucleation across the entire surface of the Si wafer. Silicon carbide on diamond can be formed by CVD depositing diamond film on a silicon wafer and then etching the original silicon wafer off to leave SiC. According to this known process SiC forms prior to diamond formation between the silicon and diamond. However, this method has little utility because of the spontaneous nucleation of either SiC or diamond on the silicon wafer. The simultaneous nucleation not only forms polycrystalline grains with grain boundaries and dislocations, but also with gaps between grains so the SiC film is not continuous.
Moreover, thick SiC wafers are a polytype with significant twinning. Hence, the advantages of SiC wafers are limited well below theoretical pure SiC wafers. For example, SiC can be used as an electrode so a GaN crystal can be symmetric with two electrodes located on opposite sides. However, this advantage disappears because LED typically use flip chips where two electrodes are oriented on the same side. Specifically, LED grown on insulating sapphire is directly bonded to the submount made of silicon which could be formed of diamond film. In such a case, no wires would be needed to connect with the electrodes, so no light will be blocked from LED. However, SiC LEDs have one electrode on the top, as well as the connecting wire. Similar problems of crystal defects (e.g. impurities, dislocations, grain boundaries, etc.), size limitations, and expense are encountered when seeking to form large single crystal substrates for electronic materials using other crystalline materials such as nitrides, diamond and the like.
Although these materials and approaches have exhibited some desirable properties, the resulting materials tend to suffer from high defect rates and poor electronic performance which limits their commercial practicability.